The lithographic process comprises several steps which include coating a thin film of the radiation sensitive resist onto a substrate which is selectively image-wise exposed to radiation that defines a pattern on the coating. In so doing, a solubility difference is induced between the exposed and unexposed areas and the developer used in the pattern development process is capable of distinguishing between the two regions and preferentially removing one kind or the other. When the exposed areas are removed by the developer, the remaining pattern is positive while the opposite leads to negative patterns.
The manufacture of integrated circuits and other patterned devices relies primarily on resist materials that enable the formation of high resolution patterns. In the search for materials and methods for formation of patterns below 0.25 microns, it is recognized that such patterns require exposure sources based on UV radiation below 248 nm, or x-ray, or e-beam. In the case of the UV radiation, it might be convenient to use excimer laser sources that produce radiation at 193 nm. Likewise, it is essential to employ resist materials suitable for use with short wavelengths sources.
It is well known in the art that the photochemical formation of carboxylic acids, be it by amplified means such as a catalytic-acidic decomposition of a tertiary butyl ester, or by the photochemical decomposition of 1,2-naphthoquinone diazides, can be employed to produce high resolution and high efficiency resists. This type of reaction is being relied upon extensively in the production of positive working resists.
An example of a positive resist suitable for short wavelength (193 nm) applications is the amplified resist described in the Journal of Vacuum Science and Technology B9 (6), p3357 (1991), by R. D. Allen et al., and also in U.S. Pat. No. 5,071,730, which describes a terpolymer based on a methacrylate backbone containing tertiary butyl esters, methyl esters or unesterified carboxylic acids side chains. Resist formulations also contain photoacid generating groups that decompose during the irradiation and cause a catalytic decomposition of the tertiary butyl esters groups, resulting in additional free carboxylic acid groups in imaged areas. These can be dissolved faster than the unexposed polymer with dilute aqueous bases. However, due to the existence of free carboxylic acid groups in non-imaged areas, the selectivity of the development is somewhat compromised.
U.S. Pat. No. 5,212,047 by Walter R. Hertler et al., assigned to E. I. Du Pont de Nemours and Co. describes another amplified resist material that provides excellent resolution and sensitivity. The composition involves polymers having recurring pendant acid labile .alpha.-alkoxyalkyl carboxylic acid ester moieties in the presence of an acid generator activated by UV, visible, x-ray or e-beam radiation. The mode of operation of this resist is similar to other amplified resists that use acid labile groups.
It is well known in the art that poly(methyl methacrylate) (PMMA) undergoes main chain scission with a low quantum yield of 0.04 to 0.14, depending on the intensity and wavelength of the radiation. Side chain scission with a minute quantum yield of 10.sup.-6 has also been reported, but is too inefficient for practical resist applications. The reported photochemistry of PMMA and its analogues, leads to solvent developable positive images and relatively low efficiency lithography. (Wayne M. Moreau, Semiconductor Lithography, p 59, Plenum Press, NY 1988).
Japanese Laid-Open Patent Application (Kodai) No. 1-244447, Sep. 28, 1989 by Nakase et al., assigned to Toshiba Corp., suggests that films of poly(2-hydroxyethyl methacrylate) and poly(2-hydroxypropyl methacrylate) can be used for pattern formation, particularly with e-beam, and developed with isopropyl acetate, an organic solvent. The mode of development of patterns therein suggests that the scission products are expected to behave in the normal mode attributable to the majority of poly acrylic and methacrylic esters, namely main chain scission.
C. F. Vernon et al. reported (Polymer International, 27, 243 (1992)), that poly(2-hydroxyethyl methacrylate) undergoes ablation when exposed with excimer laser radiation at 193 nm. Vernon et al. suggested that at high radiation fluxes (&gt;280 mJ/sq.cm.), the polymer suffers main chain scission, while at low fluxes, scission of the entire side chain 2-hydroxyethoxycarbonyl, including the moiety --COO--, occurs, with possible production of carbon dioxide.
U.S. Pat. No. 4,707,437 by John E. Walls et al., assigned to Hoechst Celanese Corporation, suggests a radiation polymerizable composition for use in the production of photographic elements such as lithographic printing plates. The composition contains a terpolymer of vinyl acetate, vinyl alcohol and a vinyl acetal, along with a photoinitiator, a diazonium salt and a photopolymerizable mixture comprising an oligomer prepared by reacting a diisocyanate compound with ethylenically unsaturated compound such as 2-hydroxyethyl acrylate, on one side and the other with a polyester polyol. The composition is used for film coating of ammonium surfaces for preparing printing plates. When exposed, the irradiated areas photo polymerize and harden, while the unexposed areas can be removed by dissolution to form a negative image.
Concerning developers and the development process, U.S. Pat. No. 4,458,008 suggests that dyestuffs that have the capacity of dyeing cotton, and others that have capacity of dyeing synthetic fibers as well as other dyes, can enhance the development of patterns in a chalcogenide photosensitive layer, when incorporated in alkaline developing agents of the usual type. The resist described is clearly an inorganic resist.
U.S. Pat. No. 5,035,982 suggests a composition for developing negative working lithographic printing plates comprising of a mixture of alkali salts of xylene sulfonic acid together with alkali salts of mesitylene sulfonic acid and an alkanol amine.
U.S. Pat. No. 3,620,726 describes a process of using colored particles to develop photodeformations in polymer films. The colorants used are fine particles pigments. The irradiation process hardens the deformable composition, along the lines of negative working resists. Following the selective hardening step, the non-polymerized sections of the film are colored to visualize the image. However, this process was not intended to produce photolithographic images.
U.S. Pat. No. 5,368,982 suggests a developing solution containing the poly(N-vinylpyrrolidone), butyl cellosolve surfactants and water.
U.S. Pat. No. 5,278,030 suggests that ethylenediamine tetraacetic acid can improve the performance of a developer.
U.S. Pat. No. 5,200,291 suggests a negative resist formulation that contains diazonium sulfonate as the light sensitive group. After exposure, the pattern is developed in a suitable organic solvent or aqueous developers that contain alkali metasilicates, phosphates or borates.
U.S. Pat. No. 5,164,286 suggests a developer for alkali developable photoresists that contains in addition to the aqueous basic solution, an amphoteric surfactant for lowering the surface tension of the developer. The preferred surfactant has an ammonium group and a carboxylic acid group in the same molecule.